stereo buffer

Writing Stereoscopic Software for StereoGraphics® Systems Using

Microsoft Windows® OpenGL

By Bob Akka StereoGraphics Corporation August 13, 1998

www.stereographics.com

Copyright ©1998-2005 StereoGraphics Corporation; All Rights Reserved.

 

Introduction | Hardware | Initializing a Window | Writing Buffers | Stereo Persective Projections | Assymetric Frustrum Projections | Camera Offset | Aesthetics | Above-Below Stereo | Ponts to Remember | Resources

1. Introduction

New PC graphics cards which include built-in support for stereoscopic buffering, along with a standardized OpenGL interface to those cards' stereo support features, now make it easier than ever to add stereo support to PC Windows-based applications. Another big advantage of these new stereo OpenGL graphics card implementations is that applications are now free to display 3D graphics elements stereoscopically without having to fill the entire screen, and in combination with other non-stereo windows and interface elements.

Stereo OpenGL buffering is supported by Microsoft Windows NT 4.0 and later. Windows 95 (Service Pack 2) and later also support stereo OpenGL buffering, though, in some cases, board manufacturers only provide stereo support with their Windows NT drivers.

This document covers the steps that you need to take to:

  • Set up for stereo development,
  • Query the graphics hardware for stereo buffering support,
  • Enable stereo buffering in a display window,
  • Write to separate stereo buffers,
  • Do stereo perspective projections, and
  • Set up your projections for excellent stereo image quality.

We strongly recommend that you also obtain "OGLPlane", a very simple stereo example program that illustrates most of the concepts that are discussed in this document. The "OGLPlane" source code and executable files are available from StereoGraphics.

Finally, this document will discuss Above-Below stereo formatting, which is used with some CrystalEyesâ and Z-Screenâ hardware implementations, and which does not require a specially equipped graphics board. Please see "2. Setting Up Your Graphics Hardware To Support Stereo" for a discussion about the difference between OpenGL stereo buffering and Above-Below formatting, and "9. Above-Below Stereo Formatting" for programming information that is specific to the Above-Below format. Sections 5 through 8, relating to stereoscopic projections, are equally applicable regardless of whether one uses OpenGL stereo buffering or Above-Below stereo formatting.

2. Setting Up Your Graphics Hardware To Support Stereo

Graphics cards that support stereoscopic buffering are able to simultaneously maintain both left- and right-eye graphics buffers. This means that, in combination with standard double-buffering (in which graphics are drawn to a "back buffer" and then swapped to the "front buffer" when all drawing is completed), the board will typically maintain four buffers when in stereo display mode.

Because of the additional memory requirements that stereo buffering demands, video boards will often not be configured to support stereo by default. After obtaining a video board that supports OpenGL stereo buffering, you will probably need to set up your display configuration to support stereo. Stereo support may be restricted to certain display resolutions and settings. Please check graphics card documentation and/or special card-specific documentation from StereoGraphics for further installation details.

If you are developing software that you want to be able to run on any video card, including those that do not support stereo buffering, you might consider Above-Below stereo formatting. In addition to the stereo viewing systems that StereoGraphics makes to work with video cards that directly support stereo buffering, StereoGraphics also makes a stereo viewing system which uses sync-doubling technology to enable stereo with just about any PC video card. This ability to use almost any PC video card is the main advantage of Above-Below stereo formatting. The disadvantages, compared to stereo OpenGL buffering, are:

  • Above-Below requires graphics windows that fill the entire screen, whereas stereo buffering systems permit windowed stereoscopy;
  • Above-Below makes it difficult to incorporate standard non-stereo interface elements, such as menus and dialog boxes, into the application while stereo viewing is active, whereas standard interface elements work fine with stereo buffering systems;
  • Above-Below stereo formatting requires graphics viewports whose pixel aspect ratios are vertically squashed; this is not an issue with stereo buffering systems;
  • Above-Below stereo formatting typically requires that the user do a "calibration" step in order for the vertical alignment to be correct; this is not an issue with stereo buffering systems.

Most companies that are currently developing software that supports stereo are targeting systems whose graphics cards include built-in support for stereo OpenGL buffering. However, if you are interested in the Above-Below method of supporting stereo, please see the detailed discussion about Above-Below towards the end of this document.

3. Initializing a Window to Enable Stereo

In order to utilize OpenGL's stereo buffering support, a graphics window must be initialized using the Windows API call, SetPixelFormat(), with the pixel format descriptor flag, PFD_STEREO, set. Here is some sample code, called from a method of the graphics window's class:

PIXELFORMATDESCRIPTOR pfd;
memset (&pfd, 0, sizeof (PIXELFORMATDESCRIPTOR)); //clear all to 0
pfd.nSize = sizeof (PIXELFORMATDESCRIPTOR);
pfd.nVersion = 1;
pfd.dwFlags = PFD_DRAW_TO_WINDOW | PFD_SUPPORT_GDI |
     PFD_DOUBLEBUFFER | PFD_SUPPORT_OPENGL | PFD_STEREO;

int iPixelFormat = ChoosePixelFormat (hdc, &pfd);
BOOL bSuccess = SetPixelFormat (hdc, iPixelFormat, &pfd);

SetPixelFormat() should be called only after a meaningful device context is available for it; it is generally good to do this right after ShowWindow() has been called.

Note that Microsoft Windows only allows SetPixelFormat() to be called once for any given window. Subsequent SetPixelFormat() calls, for any given window, are ignored. Thus, the PFD_STEREO flag must be set within a window's very first SetPixelFormat() call.

If the user's graphics configuration is not set up for stereo buffering, Windows will not allow the PFD_STEREO flag to be set. Thus, after trying to set the PFD_STEREO flag, you should always check to see if the attempted stereo enabling actually succeeded in changed the flag. Here's how:

BOOL bStereoAvailable;
// pCDC, iPixelFormat, pfd are declared in above code

iPixelFormat = GetPixelFormat (pCDC->m_hAttribDC);

DescribePixelFormat (pCDC->m_hAttribDC, iPixelFormat, sizeof
   (PIXELFORMATDESCRIPTOR), &pfd);

if ((pfd.dwFlags & PFD_STEREO) == 0) // stereo mode not accepted
   bStereoAvailable = FALSE;

else // yes we're now in stereo
   bStereoAvailable = TRUE;

What if the outcome of the above test is that stereo display is not supported by the current display configuration? A display configuration might fail to support stereo for one of several reasons:

  • The graphics card does not support stereo buffering,
  • The graphics driver does not support stereo buffering,
  • Stereo buffering is only supported at certain display resolutions or settings, or
  • Stereo buffering needs to be specially activated using some driver-specific configuration step.

OpenGL also provides a way to query the availability of stereo buffering support, using the following code:

BOOL bStereoAvailable;
unsigned char ucTest;
glGetBooleanv (GL_STEREO, &ucTest);
if (ucTest)        // yes stereo support available
   bStereoAvailable = TRUE;

else        // stereo support not available
   bStereoAvailable = FALSE;

Unfortunately, this glGetBooleanv() approach has proven unreliable with some graphics driver configurations. Thus, we recommend "querying" the graphics configuration for stereo support by actually trying to enable a window to support stereo buffering (by doing a SetPixelFormat() call with PFD_STEREO set), and then checking to see if the attempt succeeded (by checking whether the PFD_STEREO flag remains set).

4. Writing to Separate Stereo Buffers

The glDrawBuffer() OpenGL function allows you to specify which buffer subsequent OpenGL drawings and renderings should be directed to. With normal non-stereo OpenGL double-buffering, you will typically draw to the "back buffer" (glDrawBuffer(GL_BACK);), and then "swap buffers" (wglSwapLayerBuffers(hdc, WGL_SWAP_MAIN_PLANE);) in order to put what you had drawn to the back buffer onto the "front buffer", which represents the visible display.

Once you have initialized a window to support stereo buffering using SetPixelFormat() (see "3. Initializing a Window to Enable Stereo"), you can specify your drawing buffer as GL_BACK_LEFT or GL_BACK_RIGHT, which will result in subsequent OpenGL drawings and renderings appearing only in one eye's drawing buffer. After drawing to both the left and right back-buffers, do a single wglSwapLayerBuffers() call to put the back stereo buffers' contents to the front (visible) stereo buffers. Here is an example in which different colored rectangles are drawn to left and right eye buffers:

// pCDC already declared as pointer to current CDC

     // Clear both back buffers
glDrawBuffer (GL_BACK);
glClearColor(0.2f, 0.2f, 0.2f, 0.0f);
glClear (GL_COLOR_BUFFER_BIT);

     // Draw left eye view
glDrawBuffer (GL_BACK_LEFT);
glColor3b (0, 0, 127); // blue
glRectf (-0.8f, -0.8f, 0.2f, 0.2f);

     // Draw right eye view
glColor3b (127, 0, 0); // red
glRectf (-0.2f, -0.2f, 0.8f, 0.8f);

     // Put what was just drawn onto the display
BOOL bSuccess = wglSwapLayerBuffers (pCDC->m_hDC, WGL_SWAP_MAIN_PLANE);

As the above code example shows, you can still use glDrawBuffer(GL_BACK) to draw to both left and right back-buffers at once. Also note that you can use glDrawBuffer() to access the left and right front-buffers directly (GL_FRONT_LEFT and GL_FRONT_RIGHT).

5. An Introduction to Stereoscopic Perspective Projections

You now know everything you need to know to put different stuff into left-eye and right-eye buffers. Now comes the interesting part: Doing perspective projections that will result in a stereoscopic effect that is both geometrically correct and pleasing to look at.

A good quality stereo image is composed of two stereo pair elements, each of which being a perspective projection whose "center of projection" (let's use the simpler term, "camera", instead) is offset laterally relative to the other camera position.

Let's start with the mathematical representation of a simple non-stereo perspective projection. Assuming that the camera lies on the positive zaxis at (0, 0, d) (this document uses a "right handed" coordinate system), d being the distance from the camera to the xyplane, (x, y, z), projects onto the xyplane at:

(Equation 1)          

 

Thus, for example, if the camera is placed at (0, 0, 9), the arbitrary point (8, 5, 3) will project to (6, 3.75). See Figure 1 for a visual representation of perspective projection.

Figure 1: A Perspective Projection of Two Points

 

Next, we'll introduce a camera offset to the projection. To do a left-camera perspective projection, we'll offset the camera to the left by half the overall camera separation, (c/2). Except that, to make the math easier, we'll offset the entire scene to the right instead of offsetting the camera to the left. So, the left-camera projection of (x, y, z) now calculates to:

(Equation 2)          

 

And, for the right-camera projection, we'll offset the entire scene to the left; the right-camera projection of (x, y, z) now calculates to:

(Equation 3)          

 

Let's go back to the example, with the original camera at (0, 0, 9) and the arbitrary point (8, -5, -3). If we use a camera separation of 1, the left camera projects to (6.375, -3.75), and the right camera projects to (5.625, -3.75).

Notice how, in the above example, the arbitrary point ends up projecting 0.75 units to the right in the left-camera view relative to its projection in the right-camera view. If one projection is superimposed over the other in a stereo viewing system, this will result in the arbitrary point appearing in what is the optics folks call "negative parallax", meaning that it will appear to float in front of the display surface. Conversely, if a scene element appears in the left-camera view to the left of where it appears in the right-camera view, it will appear with "positive parallax", meaning that it will seem to reside somewhere behind the display surface. "Zero parallax" is what happens when the left-camera projection of a point perfectly matches its right-camera projection; a scene element projecting at zero parallax will appear to reside right at the display surface. See Figure 2 for an illustration of how one perceives negative and positive parallax effects.

Figure 2: Perception of Parallax Effects

Generally, a pleasing, well balanced stereo image will make use of both negative parallax and positive parallax, and at least some of the 3D scene will project at or close to zero parallax. Unfortunately, if one uses Equations 2 and 3 to calculate the stereo pair projections all possible 3D points will project to negative parallax. All negative parallax tends to be uncomfortable to view.

We can fix that by simply shifting the projected values leftward for the left-camera projection, and rightward for the right-camera projection. If we shift the projected points by the same amount of the original camera offset, the resulting geometry will place the original projection plane precisely at zero parallax. Scene elements originally placed in front of the projection plane will project to negative parallax, and scene elements originally placed behind the projection plane will project to positive parallax. Here are the new equations for the left eye:

(Equation 4)          

...And for the right eye:

(Equation 5)          

Returning to our example, (8, -5, -3) now projects to (5.875, -3.75) for the left camera, and (6.125, -3.75) for the right camera, shifting it into positive parallax, and making it appear behind the display surface. This makes sense, since the scene element's original z-coordinate of -3 places it behind the projection plane, whose z-coordinate is 0.

The two projections that we have derived in Equations 4 and 5 above are called "parallel axis asymmetric frustum perspective projections". Note that, even after translating the scene in one x-axis direction, and then translating the projected scene in the opposite x-axis direction, the projection axes, the camera-target vectors of the two stereo pair cameras, remain parallel. Our final step, in which we shift the projected scene, makes the frustums of the parallel projections asymmetrical, meaning that each camera's final projection shows more of the scene to one side of its axis than the other.

To summarize, the end result is a pair of perspective projections rendered from differently offset camera positions, with frustum asymmetry applied to comfortably balance the overall stereo parallax effect.

It is a common error for programmers to do stereo projections using a "toe-in" camera model. With this model, the cameras are still offset to one side or the other, but the camera-target vectors are not parallel, and converge on a single point. Some developers use this method because it is conceptually simpler and, sometimes, easier to implement than parallel axis asymmetric frustum projections. Also, with this method, a balance between positive parallax and negative parallax is achieved without the need to shift the projections as we do within Equations 4 and 5. Unfortunately, the camera toe-in approach is geometrically incorrect, and leads to some variable vertical misalignment between stereo pair elements, which can make for uncomfortable viewing. We do not recommend the "toe-in" camera model.

6. Implementing Asymmetric Frustum Projections

Now that I've explained the proper way to do stereo projections mathematically, let's talk about how to do these projections in your application. If your application already does a perspective projection, the elements that you need to add, for each of two otherwise identical stereo pair perspective projections are:

  • Center of projection (camera) offset, and
  • Post-projection shift, or frustum asymmetry

The camera offset is easily accomplished using OpenGL's glTranslate() functions (this document will use the double-precision version of the function, glTranslated()). In order to effect the pre-projection camera offset, we actually need to call glTranslated() just after the code that does the perspective projection. As in the mathematical discussion above in "5. An Introduction to Stereoscopic Perspective Projections", we will effectively translate the camera by instead translating the entire scene in the opposite direction. Here's the code (where StereoCameraOffset equals half the overall camera separation, negative for the left camera and positive for the right camera): glTranslated (-StereoCameraOffset, 0, 0);

There are a few different ways to make each stereo projection's frustum asymmetrical. One imperfect approach would be to simply apply an offset (towards the left for the left-camera rendering, towards the right for the right-camera rendering) when viewporting the regular (symmetric frustum) perspective projections. Though this approach is sometimes easy to implement, and the resulting stereo images are geometrically correct, the disadvantage is that the left and right edges of the two stereo pair element viewports will not line up with each other.

The best way to implement each stereo projection's frustum asymmetry is to integrate the frustum asymmetry into the perspective projection. The OpenGL glFrustum() function can be altered to do an asymmetric frustum projection. Here's how: glFrustum (FrustumLeft + FrustumAsymmetry, FrustumRight
    + FrustumAsymmetry, FrustumBottom, FrustumTop,
    NearClipDistance, FarClipDistance)
;

...Where FrustumLeft, FrustumRight, FrustumBottom, FrustumTop, NearClipDistance, and FarClipDistance are the arguments of the original symmetric frustum projection (since glFrustum() assumes that the camera is at the coordinate system origin, FrustumLeft should equal -FrustumRight, and FrustumBottom should equal -FrustumTop), and where FrustumAsymmetry is positive for the left-camera projection, and negative for the right-camera projection.

If your application, rather than using glFrustum(), does its perspective projections using a transformation matrix of your own creation, you can still integrate frustum asymmetry into the perspective projection matrix. Just multiply the existing projection matrix by a transformation matrix that represents a lateral translation (leftward for the left-camera projection, rightward for the right-camera projection).

A few miscellaneous details to mention:

  • glFrustum() assumes that the center of projection (camera) is at the coordinate system origin (unlike the equations used in 5. An Introduction to Stereoscopic Perspective Projections, where the camera was along the positive z-axis). Thus, you may need to apply translation transformations to your scene in order to place the camera at the origin.

  • glFrustum() also assumes that the projection axis (camera-target vector) is along the coordinate system z-axis, in the negative z direction. If that is not already the case, you will need to either apply transformations to your scene, or adapt your projection geometry. Note that the stereo camera offset should always be in a direction that is perpendicular to the original camera-target vector.

  • Finally, glFrustum() does projections such that the projection's "up-vector" (the 3D direction that will appear as "up" in the final rendering) is along the coordinate system's positive yaxis. If you are working with a projection geometry that renders uses some kind of roll angle, remember that the stereo camera offset should always be in a direction that (in addition to being perpendicular to the original camera-target vector) is perpendicular to the projection's up-vector. Also, the projection's frustum asymmetry should always be along an axis that is perpendicular to the projection's up-vector.

7. Appropriate Camera Offset and Frustum Asymmetry

In the above discussion about implementing asymmetric frustum projections, two important questions were left unanswered:

  • How much camera-offset should one use, to get a stereo effect that is strong enough to offer an effective sense of depth, yet not so strong that viewing is uncomfortable?

  • How much frustum asymmetry should be applied, in order to yield a pleasing balance between positive and negative parallax?

Both questions get into the realm of a lot of evolving research, and a certain amount of controversy. For example, StereoGraphics' own documentation used to advise "don't exceed parallax values of more than 1.5°". We know now that a person's relative acceptance and tolerance of different parallax magnitudes, on a computer screen or a projection display, is far more consistent when parallax is expressed as a percentage of viewport width, than when parallax is computed as an angular measure based on retinal disparity. Which is actually quite convenient for the software developer, since it eliminates the need to guess at variables like the user's display size and seating position.

Thus, we now advise setting up perspective projections such that the negative parallax and positive parallax effects (take off the stereo glasses to measure on-screen parallax) each fall within about 3% or so of the image's overall viewport width. Since the viewport width represents a projection plane in the 3D scene, a good starting point for offsetting each stereo camera is about 3% of the 3D scene's horizontal range. The 3D scene's horizontal range should be measured along the plane where the camera's projection frustum intersects the center of interest in the scene (which is roughly where we will probably want the plane of zero parallax to be).

So, let's say that we are rendering a mechanical part whose bounding box has a width of 100 units, and that mechanical part nearly fills the screen. You would probably want to offset each camera laterally by about 3 units. Here's the updated camera translation source code: double StereoCameraOffset = Xrange * 0.035 * UserOffsetAdjustment;
if (WhichEyeProjection == LEFT_EYE_PROJECTION)
    StereoCameraOffset = -StereoCameraOffset;
glTranslated (-StereoCameraOffset, 0, 0);

...Where Xrange equals the horizontal range of the scene along the desired plane of zero parallax. One way to derive Xrange is to get the difference between glFrustum()'s first two arguments, which represent the projection frustum's horizontal range at the near clipping plane, and multiply that difference by the ratio of the distance to the desired plane of zero parallax to the near clipping plane distance. Another way to calculate Xrange would be to multiply the distance to the desired plane of zero parallax by two times the tangent of half the horizontal field of view angle.

Also note that a "UserOffsetAdjustment" factor was thrown into the equation. It is a good idea to allow users to adjust the strength of the stereo effect to suit their preference. UserOffsetAdjustment's default value should be 1.0, and the user should be able to adjust its value downward towards 0.0 (which would result in no stereo effect at all), or upward (2.0 is usually OK as an upper limit). If your user interface provides some kind of keypad stereo adjustment, UserOffsetAdjustment should ideally be adjusted based on multiplication (i.e.: UserOffsetAdjustment *= 1.1) rather than addition and subtraction.

Next, we need to quantify the amount of each stereo projection's frustum asymmetry. Recall that in "6. Implementing Asymmetric Frustum Projections", we added a factor called "FrustumAsymmetry" (positive for the left-camera projection, negative for the right-camera projection) to each of glFrustum()'s first two arguments: glFrustum (FrustumLeft + FrustumAsymmetry, FrustumRight + FrustumAsymmetry, FrustumBottom, FrustumTop, NearClipDistance, FarClipDistance);

Further recall, from Equations 4 and 5, that the amount of frustum adjustment, when measured in the projection plane, should equal the amount of the original camera offset (though, in the opposite direction), in order for the frustum adjustment to place the projection plane at zero parallax. Thus, FrustumAsymmetry should equal -StereoCameraOffset.

However, glFrustum()'s interprets its first two parameters as xaxis boundaries, as measured on the near clipping plane. Yet, FrustumAsymmetry is being calculated based on the desired amount of frustum asymmetry in what is to become the plane of zero parallax. So, FrustumAsymmetry needs to be adjusted by the ratio of the near clipping distance to the desired zero-parallax distance. Here's the resulting source code: double FrustumAsymmetry = -StereoCameraOffset * UserBalanceAdjustment;
double n_over_d = NearClipDistance / ZeroParallaxDistance;
FrustumAsymmetry *= n_over_d;
glFrustum (FrustumLeft + FrustumAsymmetry, FrustumRight
    + FrustumAsymmetry, FrustumBottom, FrustumTop,
    NearClipDistance, FarClipDistance);

Notice that we have introduced yet another user adjustment factor, "UserBalanceAdjustment". As with the stereo camera offset, it's a good idea to let the user adjust the parallax balance to their liking. UserBalanceAdjustment's default value should also be 1.0, and the user should be able to adjust its value downward towards 0.0 (which would result in no frustum asymmetry, hence all negative parallax), or upward (2.0 is once again a good upper limit). And, like the camera offset adjustment factor, UserBalanceAdjustment should be adjusted multiplicatively (or, perhaps, via a dialog box) rather than by addition or subtraction.

Also note that the above code is set up such that UserOffsetAdjustment affects both the amount of camera offset and the amount of frustum asymmetry. This is appropriate because, if the amount of frustum asymmetry is not adjusted proportionally whenever the camera offset is changed, camera offset changes will have the side-effect of changing the scene's parallax balance.

Other stereo camera adjustment issues:

  • The horizontal frustum range (Xrange) is directly proportional to the tangent of half the horizontal field of view angle. So, if the field of view angle is changed, the stereo camera offset and frustum asymmetry both need to be recalculated.

  • The horizontal frustum range (Xrange) is also directly proportional to the distance from the camera to the desired plane of zero parallax. So, if you "dolly" the camera to get closer to an object, and want the plane of zero parallax to remain the same relative to that object's position, both the stereo camera offset and the frustum asymmetry factors will need to be recalculated. In fact, stereo camera offset and frustum asymmetry factors should be recalculated any time the distance from the camera to the desired plane of zero parallax changes.

  • Depending on the application's rendering architecture, the projection frustum may be calculated using different units from those used in the 3D scene. In such unusual cases, it may be best to keep the frustum asymmetry factor constant and independent of the camera-offset amount (though both UserOffsetAdjustment and UserBalanceAdjustment should still affect the frustum asymmetry).

8. Other Stereo Aesthetic Issues

The last few sections have discussed stereoscopic perspective projections. Some have wondered if it is possible to do stereoscopy with orthographic projections. Though it is technically possible to do stereo orthographic projections using the camera toe-in approach, the results are so uncomfortable that we strongly advise against it. Thus, developers should design their applications such that available stereo display options will always be used in combination with perspective projection.

Similarly, for the best quality stereo, the stereo pair perspective projections should use a moderately wide field of view angle (note that a narrow "telephoto" field of view angle results in a projection geometry that is quite similar to that of an orthographic projection). For the best results, we recommend a horizontal field of view angle of 50° or more.

One significant issue relating to negative parallax is a phenomenon that occurs when a scene element at negative parallax is clipped by an edge of the rendering window. The problem is that a scene element that appears to be floating somewhere in front of the display surface is occluded by an edge that is at the display surface. This results in a somewhat disturbing optical contradiction in which the occluding edge is farther away than the scene element that it occludes.

Some people firmly believe that this negative parallax edge-clipping effect must be avoided at all costs, even if that means not ever using any negative parallax at all (to do this, they place the zero parallax setting at the near clipping plane). We have found that approach to be too extreme. Eliminating negative parallax tends to result in either uncomfortable amounts of positive parallax, a flattening of the stereo effect within a narrow portion of the positive parallax range, or both. Using some balance of positive parallax and negative parallax results in stereo images that are more comfortable to view, even if that approach inevitably results in some amount of negative parallax edge-clipping.

How you should deal with the negative parallax edge-clipping issue depends on the nature of your application. In a mechanical CAD application where the scene elements of interest will usually not be clipped by the window edge, it is usually safe to place the zero-parallax plane at or just in front of the object's center. However, with a "flythrough" type of application, it is often best to set the zero-parallax distance such that most (though not all) of the scene projects to positive parallax.

Finally, it should be mentioned that very high contrast values in a stereo image will often result in "ghosting", in which some of one eye's view appears as a "ghost" in the other eye's view. This tends to be caused by CRT phosphor persistence (ghosting can also be an issue when using projection displays). Green tends to be the worst offender. The workaround to this problem is to avoid extreme amounts of contrast if at all possible. Use a gray background instead of a black background (even dark gray is better than black), and if possible, try to avoid using bright white, green, and cyan scene elements against a dark background.

9. Above-Below Stereo Formatting

In addition to supporting the OpenGL stereo buffering standard, StereoGraphics also makes a stereo viewing system that permits full-screen stereo viewing with almost any PC graphics card. Stereo software designed for this type of stereo system needs to use Above-Below stereo formatting. (Note that this section is not relevant to you if you are developing software using OpenGL stereo buffering. For more information about the differences between these two stereo hardware systems, please see "2. Setting Up Your Graphics Hardware To Support Stereo".)

The Above-Below system's stereo emitter includes circuitry which, when in stereo display mode, doubles the frequency of the vertical sync signal. This causes the display to refresh at twice its normal rate, which has the effect of vertically stretching the screen display. At this doubled display frequency, whatever was previously on the top half of the display, and whatever was previously on the bottom half of the display, each take up the full display on alternate refreshes.

Thus, to put up a stereo image which will appear stereoscopically using this hardware system, you need to viewport the left camera's rendering to the top half of the overall display, and viewport the right camera's rendering to the bottom half of the overall display. Since the sync-doubling circuitry stretches everything vertically, each rendering should have a 1:2 pixel aspect ratio such that everything originally appears vertically squashed.

There is one more important detail to Above-Below stereo formatting: Normally, every vertical scan of the display includes hundreds of visible horizontal scan lines, followed by 20 to 50 invisible ones (the "blank interval"). When sync-doubling, we need to insert a second blank interval (draw it as a black field) across the middle of the original display. The two stereo pair viewports end up with the remainder of the display, each getting a little less than half of the original vertical display resolution.

For example, if the original display resolution is 1024x768, and the blank interval thickness is 40, each stereo pair viewport ends up with half of the remainder of the vertical resolution, (768-40)/2, or 364. Thus, each stereo pair viewport will have a pixel size of 1024x364, and the two stereo pair viewports will be separated by a black blank interval region whose pixel size is 1024x40. The left-camera projection would be mapped to the top viewport (pixel-rows 0 through 363), the right-camera projection would be mapped to the bottom viewport (pixel-rows 404 through 767), and the new blank interval would appear exactly mid-display (pixel-rows 364 through 403).

In order for the two stereo pair viewports to be positioned with proper vertical alignment, the newly created blank interval must have a vertical thickness that perfectly matches the original blank interval. Since blank interval thickness varies, depending on the user's graphics card, display resolution, and other display settings, Above-Below stereo software must be designed to accommodate different blank interval thicknesses. This usually means providing the user with some kind of calibration screen, in which the user lines up two arrows while in stereo mode.

Remember: Sync-doubling affects the entire display, including window borders, menu bars, the Windows Taskbar, other application windows, etc. For this reason, an Above-Below application window needs to fill the entire display, blocking out all other application windows including the Windows Taskbar. Additionally, an Above-Below application window should neither have a border, nor standard title bars, menu bars, or status bars. Interface design using Above-Below stereo formatting can be a challenge.

Other Above-Below programming notes:

  • With the StereoGraphics sync-doubling hardware that supports Above-Below stereo formatting, the user toggles stereo display mode on and off manually using a physical switch. Thus, stereo display mode switching is not controlled by software.
  • Sections 5 through 8, relating to stereoscopic projections, apply to Above-Below stereo formatting, as well as OpenGL stereo buffering.

10. Points to Remember

  • With OpenGL stereo buffering, the necessary elements are:
    - Check for stereo display configuration availability, and enable stereo buffering support when initializing each graphics window.
    - Render two asymmetric frustum perspective projections, one to each of the two stereo back-buffers. Then swap buffers to put the stereo pair on the display.
    - Offset each stereo camera using a lateral translation (glTranslate()). Stereo projection axes should remain parallel to the original projection axis.
    - Balance the stereo parallax by making the perspective projection frustums asymmetrical. This can be done by changing the first two arguments of glFrustum(), or by altering the transformation matrix that does the projection.

  • Use enough stereo camera separation to yield a pleasing stereo effect, but not so much as to be uncomfortable. In general, on-screen parallax should be limited to about 3% or so of viewport width, for both negative and positive parallax effects. For the best stereo effect, use a wide-angle perspective projection, with a field of view angle of 50° or more.

  • Parallax effects should be balanced such that there is some negative parallax, and some positive parallax. To do this, apply the right amount of projection frustum asymmetry to put the plane of zero parallax near the center of interest in any given scene. (For some applications such as "flythrough" applications, it may be desirable to use more positive parallax than negative parallax, in order to reduce the amount of negative parallax that is clipped by the window edges.)

  • Stereo projection settings should not be fixed quantities. Recalculate the camera separation and frustum asymmetry any time that the field of view angle or the camera-to-target distance changes. Stereo projection setting calculations should also include user-adjustable factors.

  • Above-Below formatting is another approach to doing stereo display on a PC. OpenGL stereo buffering and Above-Below formatting use different StereoGraphics stereo display hardware.

11. Resources

As previously mentioned, many of the concepts discussed in this document are illustrated in the example program, "OGLPlane", which is available from StereoGraphics. Another simpler example program, "RedBlue", illustrates stereo OpenGL buffering without doing any stereo projections (it simply draws colored rectangles to each stereo buffer). Other example programs may additionally be available at our website, http://www.stereographics.com , and in the "Developers" directory of our ftp site, ftp.stereographics.com. StereoGraphics developer support can also be reached at support@crystaleye.com, or 415-459-4500.

posted on 2007-05-03 18:38 zmj 阅读(1654) 评论(0)  编辑 收藏 引用


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